Homosalate CAS 118-56-9

Introduction to Homosalate

Homosalate is a clear, colorless to pale-yellow liquid, the salicylate ester of 3,3,5-trimethylcyclohexanol. The aromatic salicylate ring absorbs UVB radiation across 295–315 nm with a peak near 306 nm, while the bulky trimethylcyclohexyl group drives strong lipophilicity (log P ≈ 6.1). Formulators use it as a UVB filter in sunscreen emulsions and as the oil-phase solvent that keeps solid filters such as avobenzone dissolved.

The low molar extinction coefficient (~4,500 L·mol⁻¹·cm⁻¹ at 306 nm) is the defining constraint: even a 10% dose contributes only moderate SPF, so homosalate must be paired with high-extinction filters to reach SPF 30+. One practical caution: at 10–15% loads the oil phase turns heavy and can break O/W emulsions; keep total oil phase below 30% or balance with lighter emollients and polymeric emulsifiers.

Key Features of Homosalate

• UVB absorption at 295–315 nm. The salicylate chromophore peaks at ~306 nm, covering the erythema-effective UVB range and keeping the filter profile predictable in combination systems.
• Solvent for solid filters. Dissolves crystalline filters such as avobenzone in the oil phase, preventing recrystallisation in bulk storage and on skin during wear.
• Water-resistant film former. Log P ≈ 6.1 keeps the molecule in the oil film after water immersion, supporting 80-minute water-resistance claims when paired with film formers.
• Limited skin penetration. The 262.34 g/mol weight and bulky cyclohexyl group restrict stratum corneum permeation, underpinning favourable safety assessments.
• Photostable ester backbone. With no non-aromatic double bonds, UV exposure neither isomerises nor cleaves the molecule, so absorbance holds during sun exposure.

Homosalate Chemical & Physical Properties

Applications of Homosalate

Sunscreen emulsions (O/W and W/O): Dosed at 5–15% where regulations allow. The salicylate ring absorbs UVB at 295–315 nm; combined with UVA filters it builds broad-spectrum systems. Under EU Regulation 2022/2195, face products (excluding propellant sprays) cap at 7.34%.

Solvent and carrier for solid UV filters: Used at 3–8% of the formula to hold 1–3% avobenzone in solution. The ester environment suppresses needle formation during cold storage at 5 °C and keeps the filter optically active on skin.

Daily-wear SPF moisturisers and primers: Added at 2–6%. Contributes UVB protection while plasticising the emollient phase, giving a smooth spread without the white residue of mineral filters.

Lip care and anhydrous stick products: Formulated at 2–6% in wax-based sticks. The liquid ester lowers the brittle point of the wax matrix and delivers uniform UVB coverage across the applied film.

Storage & Safety Precautions for Homosalate

Storage conditions. Store in sealed HDPE drums or IBC totes at 10–30 °C, protected from light. Viscosity rises sharply below 10 °C; warm drums to 25–30 °C and homogenise before pumping. Keep away from strong oxidisers and strong alkalis—the ester bond hydrolyses above pH 10 at elevated temperature, releasing salicylic acid and 3,3,5-trimethylcyclohexanol.

Safety and PPE. GHS: Not classified as hazardous. Mild eye irritant. Wear standard nitrile gloves and safety goggles during drum transfers. Spills are non-toxic but extremely slippery; contain with inert absorbent and wash with warm soapy water.

Transport classification. Non-DG. Not regulated under IMDG, IATA, or ADR. Ship as standard general cargo. Ensure drum bungs are sealed to prevent moisture ingress during ocean freight.

Practitioner note: For incoming QC, assay by GC-FID with an internal standard and confirm identity by refractive index (1.516–1.518 at 20 °C). An RI below 1.514 signals dilution with unreacted 3,3,5-trimethylcyclohexanol; titrate free salicylic acid and reject above 0.5%. Colour may drift from water-clear to pale yellow between lots without affecting UV absorbance.

Homosalate FAQ

Q: What is the maximum dose of Homosalate (CAS 118-56-9) in EU and US sunscreens?

A: In the EU, Regulation 2022/2195 limits homosalate to 7.34% in face products (excluding propellant sprays), enforceable from 2025. The US FDA OTC monograph allows up to 15%. For EU-bound face creams, cap at 7.34% and rebuild SPF with companion filters; for US-only products, 10–15% remains compliant.

Q: Why does 10% homosalate alone fail to reach SPF 30?

A: The salicylate chromophore has a low molar extinction coefficient (~4,500 L·mol⁻¹·cm⁻¹ at 306 nm). A 10% dose alone delivers roughly SPF 6–8. Treat homosalate as a booster and solvent, not the primary filter: pair it with high-extinction UVB and UVA filters to reach SPF 30+ without an oily, unstable oil phase.

Q: Will homosalate keep avobenzone dissolved during cold storage?

A: Yes at typical loads. With homosalate at ≥5% of the formula, 1–3% avobenzone stays in solution down to 5 °C. Below that, run a 7-day crystallisation check at 5 °C; if needles appear, raise the homosalate share or add a co-solvent ester. Always pre-dissolve avobenzone in the homosalate before adding the remaining oil phase.